1. Field of the Invention
The present invention relates to a fixing device used for an electrophotographic image forming apparatus such as a copier, a printer or a facsimile, and more specifically to a fixing device for fixing an unfixed image formed on a recording member by noncontact means such as laser beam irradiation means, and an image forming apparatus including the same.
2. Description of the Prior Art
An electrophotographic image forming apparatus (for example, a copier, a printer, a facsimile or the like) is provided with a fixing device for fixing a toner image formed on a recording member (recording sheet, or the like) by heating and fusing. As an example of such a fixing device, as shown in the following Patent Literature 1, a roller-pair system fixing device comprised of a fixing roller and a pressing roller is known.
The fixing roller is a roller member having an elastic layer formed on the surface of a hollow cored bar made of metal such as aluminum, and has a configuration in which a halogen lamp is arranged inside the cored bar as a heat source. A temperature control device then performs on/off control of the halogen lamp based on a signal that is output from a temperature sensor provided on the surface of the fixing roller, thereby controlling temperature on the surface of the fixing roller.
The pressing roller is a roller member having a heat-resistant elastic layer such as silicone rubber provided on the cored bar as a coating layer. Such a pressing roller is in press-contact with a peripheral face of the fixing roller, having a nip area that is formed with the fixing roller therebetween due to elastic deformation of the above-described elastic layer of the pressing roller.
With the above-described configuration, in the fixing device, a recording sheet on which an unfixed toner image is formed is held in the nip area between the fixing roller and the pressing roller, and both of these rollers are rotated, thereby conveying the above-described recording sheet, as well as fusing and fixing on the recording sheet the toner image on the recording sheet by heat of the peripheral face of the fixing roller.
However, in a conventional roller-pair system, the fixing roller and the pressing roller right after supplying power are at room temperature in the case of a power-off state of the device for a long period of time, thus needed to be increased to a predetermined temperature and required to have a warm-up period. Moreover, in a stand-by state where printing operation (image forming operation) is not performed, it is necessary to maintain the surface of the roller at the predetermined temperature, thus having to be heated all the time even when the printing operation is not performed. Therefore, wasted energy is consumed other than the printing operation.
Consequently, as a method of effectively fixing only a toner without consuming wasted energy, a fixing device has been proposed for fixing a toner using laser power as described in the following Patent Literature 2.
According to Patent Literature 2, a plurality of lasers are used to directly heat a toner. Thus, a plurality of lasers are used so that fixability which is insufficient only with one weak laser is improved. Thereby, the surface of the fixing roller is not needed to be maintained at a predetermined temperature, and wasted energy is thus not consumed. It is also described that it is possible to use an inexpensive laser by low output, thus making it possible to also simplify the whole of the device.    Patent Literature 1: Japanese Patent Application Laid-Open Hei 11 No. 38802    Patent Literature 2: Japanese Patent Application Laid-Open Hei 7 No. 191560
Like Patent Literature 1, in the case of a contact-fixing method for fixing a toner by heating and pressing including widely-used roller fixation, belt fixation and the like, for adhesion between the toner and a recording sheet, the toner which is fused by heating is subjected to pressure with a roller nip portion (contact portion of a roller which serves as an upper roller) so as to have promoted diffusion (adhesion force) to paper for adherence, thereby attaining favorable fixability.
However, in the case of a laser fixing device as described in Patent Literature 2, there is no pressing means, and it is thus impossible to impart diffusing strength to a recording sheet. Diffusion (adhesion force) of a toner to a recording sheet is controlled by factors such as viscosity when a toner is fused, wettability to a recording sheet and the like. Accordingly, even when a toner is fused to have lowered viscosity, the toner has no diffusion (adhesion) to a recording sheet in the case of having inferior affinity to the sheet in some cases.